1. Field of the Invention
This invention relates to a method of and an apparatus for mirror-like polishing a chamfer of a semiconductor single crystal wafer (hereinafter referred to as wafer having an orientation flat).
2. Description of the Prior Art
Wafer chamfer mirror-like polishing of a wafer comprising a semiconductor single crystal, is made for such purposes as preventing dust generation and coping with liquid pool when washing the wafer.
Such wafer, as shown in FIG. 5, has its periphery formed with an orientation flat part W.sub.2. At corners W.sub.3 between the intrinsic peripheral part W.sub.1 and orientation flat part W.sub.2, the curvature radius r.sub.3 is very small, and in this locality the relative curvature radius with respect to the buffing wheel for mirror-like polishing the wafer 1 is extremely small compared to the other localities. Therefore, with a constant pressing pressure the contact pressure p is very high at the corners W.sub.3. In the meantime, when the wafer is rotating at a constant rotation number, the speed of movement of the point of contact between the wafer chamfer and the buffing wheel is greatly reduced at the corners, thus extending the process time at this locality. For the above reasons, the mirror-like polishing of the corners W.sub.3 that is done under the same mirror-like polishing conditions (i.e., wafer rotation speed, pressing pressure between the buffing wheel and wafer, rotation speed of the buffing wheel, etc.) as for the intrinsic peripheral part W.sub.1 and orientation flat part W.sub.2, results in excessive wear or wedging of the buffing wheel at the corners.
The capacity C of wafer chamfer mirror-like polishing is obtained from the following general approximation equation EQU C=a.sub.1 pV.sub.b T
where a.sub.1 is a constant (a.sub.2, . . . , a.sub.n appearing in the following being the same), p is the contact pressure, V.sub.b is the relative speed .varies.N.sub.b (N.sub.b being the rotation speed of the buffing wheel), T is the contact time .varies.1/N.sub.S (N.sub.S being the rotation speed of the wafer). Hence, EQU N.sub.S =a.sub.2 pN.sub.b /C
As for p (approximated by two-circle contact between wafer circle and buffing wheel circle) EQU p=a.sub.3 {F(1/R1+1/R2)}.sup.1/2 (F being the pressing pressure).
Hence, EQU N.sub.S =a.sub.4 N.sub.b {F(1/R.sub.1 +1/R.sub.2)}.sup.1/2 /C
Assuming that a.sub.4, N.sub.b, C and F are constant, we have EQU N.sub.S =a.sub.5 {(1/R.sub.1 +1/R.sub.2)}.sup.1/2
where R.sub.1 (diameter of the buffing wheel) is constant. Taking R.sub.2 (diameter of the wafer) as a variable, relation as shown in Table 1 below is obtained in connection with the showing in FIG. 5.
TABLE 1 ______________________________________ Wafer peripheral position W.sub.2 W.sub.1 W.sub.3 ______________________________________ R.sub.2 Large (.infin.) Medium (r.sub.1) Small (r.sub.3) N.sub.S Small Medium Large ______________________________________
When the pressing pressure F (Kgf) of the buffing wheel is constant, the area of contact between the wafer and the buffing wheel is small with a small relative curvature radius of the wafer and large with a large relative curvature radius.
It is thus possible to control the wafer chamfer mirror-like polishing capacity C through control of p, N.sub.S and N.sub.b noted above.
A technique of controlling the excessive wear of the corners of wafer through control of the contact pressure p between the wafer and buffing wheel while controlling the wafer chamfer mirror-like polishing capacity C, is shown by the applicant in Japanese Laid-Open Patent Publication No. 6-155263.
In this technique, when mirror-like polishing the wafer chamfer, the mirror-like polishing capacity C is made uniform for the orientation flat part, intrinsic peripheral part and corners by varying the pressing pressure between the wafer and buffing wheel according to a wafer position detection signal from wafer position detecting means, which makes a determination as to whether the wafer mirror-like polishing position corresponds to the orientation flat part, intrinsic peripheral part or corner.
The curvature radius of the corner is about 2 mm, and with an 8" wafer (with a radius of about 100 mm) which has the orientation flat part W.sub.2 as noted above, the processing time of the corner W.sub.3 is usually reduced to a couple of seconds by setting the wafer rotation speed to about one minute per one round.
However, when the wafer mirror-like polishing position goes from intrinsic peripheral part W.sub.1 to corner W.sub.3 and from corner W.sub.3 to orientation flat part W.sub.2, the mirror-like polishing capacity C is varied in these localities as shown by the solid plot in FIG. 4(B) unless the pressing pressure is quickly raised and lowered. In the above technique of controlling the pressing pressure between the wafer and buffing wheel, the pressing pressure generating means employs an air cylinder which is inferior in the response property. Therefore, a response delay is generated as shown by the dashed plot in FIG. 4(B). This frequently results in the occurrence of excessive mirror-like polishing or wedging into the buffing wheel particularly at the corner W.sub.3.
The follow-up property can be improved by using an oil hydraulic cylinder. In wafer mirror-like polishing, however, oil is undesired because it causes impurity introduction.
It is possible to control the rotation speed Nb of the buffing wheel for the control of the mirror-like polishing capacity C. However, the buffing wheel is rotated at a high rotation number and has a high moment of inertia. The high momentum thus generated deteriorates the response property, so that it is difficult to obtain fine and accurate control.